The present invention has for its object a method for the vacuum thermoforming and simultaneous piercing of articles of thermoplastic material provided with at least one hole, according to which method a sheet of thermoplastic material is softened by heat and it is caused to adhere against a die and against at least one pointed projection projecting from the die, under the action of vacuum generated between the sheet and the die by means of suction bores provided in the die itself.
The known methods of the above type are based on the fact that, upon piercing of the sheet at the moment of its vacuum thermoforming, there is established, between the sides of the pointed projection and the sheet pierced by same, an airtight seal which is sufficient in order to avoid the loss of vacuum generated between the sheet and the die and therefore to ensure the positive drawing by suction of the softened sheet into the die itself. A vacuum-forming method and die of this type are illustrated, for example, in the U.S. Pat. No. 3,308,213 to Carrigan et al. Practically, however, the above airtight sealing effect has proven to be sufficient only for relatively thin pointed projections presenting a very small diameter, which can be assimilated to needles, and which serve for piercing relatively small holes in the articles of thermoplastic material formed by the forming die, such as for venting holes in closure lids. In the case that it is desired to obtain larger diameter holes, such as ventilation holes provided at the bottom of the pockets of pocket trays used for fruit and vegetable products, the piercing of the sheet of plastic material permits a remarkable loss of the vacuum generated between the softened sheet and the forming die, and consequently it does not permit the softened thermoplastic sheet to be completely drawn by suction and to adhere against the bottom and side walls of the forming die.
The invention has for an object to eliminate the above mentioned inconveniences, and it provides an improved vacuum forming and piercing method and device which can be utilized whenever articles made of thermoplastic material there be pierced, simultaneously with their vacuum forming, large diameter holes, such as holes presenting a diameter from 3 to 12 millimeters, or even more.
For this purpose, in accordance with the invention, a softened sheet of thermoplastic material, prior to being pierced by the pointed projection, is caused to adhere against the die along an annular zone all around the base of the pointed projection itself. In this manner, prior to being pierced by the pointed projection of the softened sheet drawn into the die, there is formed an annular airtight sealing zone between said sheet and said die, around the base of the pointed projection, which annular sealing zone ensures the normal sucking and consequently a perfect adherence of the sheet against the whole surface of the die located outside of said annular sealing zone. As a consequence, while being drawn by suction inside the die, the sheet is pierced by the pointed projection and stretched so as to slide down along the said pointed projection and to perfectly adhere also around the base of the said pointed projection, at the interior of the said annular sealing zone.
The method according to the invention may be carried out by employing any suitable device for the vacuum thermoforming. More particularly, in the case of vacuum thermoforming with simultaneous piercing of pocket trays or cup-shaped containers adapted for containing fruit and vegetable products, in which the pocket of the tray presents at least one ventilation hole obtained in its bottom, the invention contemplates also a device consisting of a forming die presenting at least one cavity and suction bores arranged at the bottom of said cavity, and which is characterized by the feature that from the bottom of said cavity, in the center thereof, there projects outwardly at least one pointed projection presenting a height which is remarkably less than the depth of the cavity, while the suction bores are arranged in the peripheral zone of the bottom of the cavity.
The above and other characteristic features of the invention, and the advantages deriving therefrom will appear evident from the following description of a preferred embodiment, which has been diagrammatically illustrated in the attached drawings.